[peri.car.di.um] n, pl -dia [ME, fr. ML, fr. Gk perikardion, neut. of perikardios around the heart, fr. peri- + kardia heart--more at heart] (15c) 1: the conical sac of serous membrane that encloses the heart and the roots of the great blood vessels of vertebrates
2: a cavity or space that contains the heart of an invertebrate and in arthropods is a part of the hemocoel